This invention relates to mesophase pitches. These pitches show an ordered liquid crystalline structure wherein the aromatic pitch molecules associate to form a somewhat sheet-like arrangement. The ordered liquid crystalline structure of mesophase pitch makes such pitches especially suitable for forming ordered structural artifacts such as pitch carbon fibers.
It has long been known that carbon fibers can be produced from mesophase pitches. These fibers have excellent properties suitable for commercial uses because of their light weight and thermal and electrically conductive properties, as well as being strong and stiff. These fibers are normally chemically and thermally inert, and usually find use as reinforcements in composites such as aerospace applications.
Pitch carbon fibers are generally of two types. One type of carbon fiber is produced from isotropic pitches which exhibit little molecular orientation and have relatively poor mechanical properties. However, the second type of carbon fiber is produced from mesophase pitch, (or optically anisotropic pitches) which exhibit highly aligned molecular orientation providing excellent mechanical properties and extremely high modulus values.
Various processes are known to produce mesophase pitches. All known processes have two common elements, one being a growth reaction wherein relatively small aromatic molecules are converted into larger mesophase-size aromatic molecules known as mesogens. The second element is a concentration of these mesogens to form mesophase pitch.
Concentration involves removal of smaller aromatics and sometimes includes removal of excessively large aromatics. Techniques well known for accomplishing these end results include solvent extraction, distillation, gas stripping and phase separation. We have discovered supercritical solvent extraction can also be used.
Mesophase pitches suitable for spinning into pitch carbon fibers have from 40 to 100 percent optical anisotropic content and from 0 to near 100 percent quinoline insolubles. Suitable pitches should form a homogenous melt. Suitable pitches having a melting point in the range of 250.degree. C. to 380.degree. C. have been reported. Spinning into fibers becomes a problem because of pitch thermal instability above about 350.degree. C. and therefore pitches melting at 310.degree. C. to 350.degree. C. or lower are preferred.
As spun fibers melt at about the same temperature as the spinnable pitch. These fibers require oxidative stabilization to become infusible before they can be converted to carbon or graphite fiber at temperatures of 1000.degree. C. or higher. The stabilization step is highly exothermic. Great care must be taken to control stabilization so that the treatment is uniform and so that partial melting does not occur. The required slow careful stabilization is expensive and adds significantly to the cost of pitch based carbon fiber.
It would therefore be of great benefit to provide an anisotropic pitch which is fluid at much lower temperature than conventional mesophase. It would also be of great benefit if the lower melting anisotropic pitch was much higher melting after spinning. Other objects will become apparent to those skilled in this art as the description proceeds.
For the purposes of this specification and claims the following terms and definitions are used:
"Anisotropic pitch" or "mesophase pitch" means pitch comprising molecules having aromatic structures which through interaction are associated together to form ordered liquid crystals, which are either liquid or solid depending on temperature. PA1 "Fibers" means continuous lengths of fiber capable of formation into useful articles, and comprises both continuous filaments and fibrils. PA1 "Fibrils" means small filaments of varying lengths. PA1 "Isotropic pitch" means pitch comprising molecules which are not aligned in ordered liquid crystals. PA1 "Mesogens" means molecules which when melted or fused form mesophase pitch and comprise a broad mixture of large aromatic molecules which arrange upon heating to form liquid crystals. PA1 "Oriented Molecular Structure" means the alignment of aromatic pitch molecules in formed carbon-containing artifacts, wherein said alignment provides structural properties to the artifact. PA1 "Petroleum pitch" means to the residual carbonaceous material obtained from the catalytic or thermal cracking of petroleum distillates or residues. "Petroleum coke" means the solid infusible residue resulting from high temperature thermal treatment of petroleum pitch. PA1 "Pitch" as used herein means substances having the properties of pitches produced as by-products products in various industrial production processes such as natural asphalt, petroleum pitches and heavy oil obtained as a by-product in the naphtha cracking industry and pitches of high carbon content obtained from coal. PA1 "Pitch oils" means those portions of a pitch that can be distilled or evaporated by such techniques as vacuum distillation, wiped film evaporation or sparge gas stripping. Most pitches including mesogens, pseudomesogens, and solvated mesophase contain pitch oils. PA1 "Pseudomesogens" means materials which are potentially mesophase precursors, but which normally will not form optically ordered liquid crystals upon heating, but will directly form a solid coke upon heating, such that there is no melting or fusing visible. PA1 "Solvated mesophase" means a material having a mesophase liquid crystalline structure which contains of between 5 and 40 percent by weight of solvent in the liquid crystal structure, the remainder comprising of mesogen or pseudomesogen pitch, and which melts or fuses at a temperature of at least 40.degree. C. lower than the pitch component when not associated with solvent in the structure. PA1 "Solvent Content" when referring to solvated mesophase is that value determined by weight loss on vacuum separation of the solvent. In this determination, a sample free of entrained or trapped solvent is obtained. The sample is accurately weighed, crushed and then heated to 150.degree. C. during 1 hour in a vacuum oven at 5 mm pressure. The sample is then heated to 360.degree. C. during 1 hour and held at 360.degree. C. under vacuum for 1/2 hour. The weight loss or difference in weight times 100 divided by the original sample weight is the percent solvent content.